Every year before Easter, Santorini residents refresh the paint on their houses to get this clean look that you often see on the pictures from the Cyclades Islands. Houses in this part of Greece are traditionally covered with hydrated lime and water. It's a cheap and very effective paint, but it needs to be reapplied at least once a year. The coating also has antimicrobial properties and was used for centuries to fight epidemics. Sometimes people add a bit of blue color in the whitewash to reduce its brightness. Blue and white colors also are symbolic since they represent the Greek flag.
White color reflects light so it helps houses to stay cool in the summer. Many buildings are very old and don't have any air-conditioning. Some houses are whitewashed up to three times a year, but most residents do it once a year, during spring. Every person is responsible for painting their own home.